An autoblock is, as the name suggests, an automatic block of an IP address set by the MediaWiki software. Autoblocks are the result of an attempt to edit Wikipedia from an IP address recently used by a blocked user.

Each time a user edits Wikipedia, the IP address used to connect to the site is recorded by the MediaWiki software that powers Wikipedia. A log of IP addresses used by every user is kept privately, accessible only by users with checkuser access to the MediaWiki software. When a blocked user attempts to edit the site, the IP from which they are editing is "autoblocked," so that they may not make the same edit anonymously or under a different user name.

An administrator can disable autoblocking when blocking a user. This is done automatically by MediaWiki for common dynamic IP ranges such as those used by AOL. (A list of such automatically exempt IP ranges can be found at MediaWiki:Autoblock whitelist). However, if an administrator is unaware that the user is editing from a dynamic IP pool, autoblocker is usually left enabled. Older indefinite blocks, dating from before the autoblock exemption whitelist and the option to disable when blocking, may also trigger autoblocks (while these can be fixed by unblocking and re-blocking the user, it is tedious to do so unless absolutely necessary). When this happens, users may be autoblocked as the result of a block on another user, often one using the same ISP, when they have personally done nothing wrong. This is referred to as "collateral damage".

User gets blocked

User accesses site

Software records IP address

Software automatically blocks that IP address for 24 hours

Any other IP address used by the blocked user is then blocked as well

There is an internal autoblock expiry time variable, which is set to 24 hours, meaning that autoblocks only last for 24 hours. However, in the case of dynamic IP pools, this may affect hundreds of users before the block expires. In the case of an indefinite block, autoblocks may continue to be set by the software weeks or months after the initial block has been set. (It is not uncommon to find autoblocks listed on Special:Ipblocklist where the initial indefinite block was issued in 2003 or 2004.) Users who find themselves autoblocked accidentally should contact the administrator that issued the initial block and request an unblock. Unblocks may also be requested in the #wikipedia IRC channel on freenode.

It is important for users to understand that administrators do not set autoblocks; once they have blocked a user with autoblocking enabled, autoblocks are set by the MediaWiki software. Autoblocks do not appear in administrators' block logs, and they are not notified of them. This is a necessary consequence of the need, stated in Wikipedia's privacy policy, to keep logged-in users' IP addresses private. While the IP address responsible for every edit is recorded by the MediaWiki software, this cannot be accessed, even by administrators and even when the user is blocked. The only circumstances in which a user may be associated with an IP address are certain policy violations detailed by the checkuser policy; even then, access to the checkuser tool is granted only to a small group of trusted users, not administrators in general.

When IP addresses are autoblocked, they appear in Special:Ipblocklist (but not in the admin's block log) with a special mask that prevents the IP from being seen. Autoblocks register on Special:Ipblocklist and in the blocked user text with the name of the admin that set the original block, but that admin is not notified that a block has been placed. For example:

User:Steven, who also uses AOL, signs on to Wikipedia, and is assigned the IP address last used by George.

The MediaWiki software, detecting the use of the IP by Steven, and assuming it to be George, issues a 24 hour block in Susan's name. Susan is not notified of the block.

Steven, misunderstanding the nature of an autoblock, angrily demands to know why Susan has blocked him.

Much WikiDrama ensues.

It is helpful for all involved, especially AOL users, who are often chronically autoblocked by collateral damage, to remain patient and remember that it is the software, not the admin who is unfortunate enough to have his or her name appear on the block, who is responsible. Equally important is for admins to check Special:Ipblocklist regularly, and unblock all autoblocks from a particular user if more than two autoblocks are set in rapid succession. (If more than two IPs are autoblocked within seconds/minutes of each other, it is a good indication that it is a dynamic IP pool and the blocks are collateral damage.) Also, autoblocked users should email the admin before posting angry messages to AN/I or the mailing list, as 1) it isn't the admin's fault, and 2) it can quickly be taken care of without the need for hurt feelings.

Finally, users should know that autoblocked users are often able to edit pages in the talk namespace, but not the main namespace. For instance, an autoblocked user is often able to edit Talk:Foo but not Foo itself. In this case, it is often possible for the user to leave a note for the admin directly on the admin's talk page. Make sure to note that it is an autoblock, as the admin may mistake your ability to post to his or her talk with being unblocked. It is imperative, whether you post to a talk page or contact an admin via email, that you include the IP address or username that is reported in the block text, as the admin has no way of determining which IP is autoblocking you.